The sensory relay in the mammalian thalamus is being studied by combined light and electron microscopic methods, in adults and during early stages of post-natal development. Synaptic patterns of contact established within the lateral geniculate nucleus and other thalamic nuclei are being investigated by staining or filling the cells and afferent axons and by studying these marked profiles first on thick sections light-microscopically, and then, using the same material, on thin sections electron-microscopically. This method is being used on cats and on kittens and, in addition, the development of the lateral geniculate nucleus of ferrets, which are born before any of the geniculate laminae form, is being studied by the same methods and by classical neuroanatomical techniques. In relation to these developmental studies, the "sprouting" of retinogeniculate axons from their normal territory into experimentally denervated territory is being investigated in cats and ferrets.